I couldn't find your quote anywhere except on Spanish Wikipedia , where the sentence is quoted without any sources. Had a brief look into Suetonius' "The Twelve Caesars" where I couldn't find any trace of this . Could you give some more context if at all possible.

I had checked some Roman historians but I didn't obtain any satisfying result. The only sources I found are Spanish websites, but it seems that they quote each other without adding information. Anyway it's an appealing sentence even though it be a hoax.

It sounds apocryphal to me. There is no mention of any such remark in either Suetonius or Plutarch.
Caesar was then eighteen and Sulla was dictator and in the process of putting his political enemies to death.
As the nephew of Marius, Caesar was targeted and forced to flee for his life, not returning to Rome till Sulla was dead.
When Sulla's relatives and friends were advising him not to kill the young man, Sulla is reported to have said, 'In Caesar there are many Mariuses'. This saying by Sulla is vouched for by Suetonius and Plutarch, but the alleged remark of Caesar is not reported. If it appears in any later writers it seems likely to be an invention.

This spanish site says flat out that it is a fabrication not found in any ancient text. On the other hand, it seems to be a well-known quotation in the Spanish language world. That suggests to me that it originated in some sort of Spanish political polemic of the past.

It takes the heroic-enough known facts, when Caesar dared to defy the Dictator Sulla by refusing to divorce his wife, and turns them into an object lesson for future generalissimos everywhere.

This spanish site says flat out that it is a fabrication not found in any ancient text.

Click to expand...

As I read it, it says that the saying appears to be a fabrication, as it has not been possible to verify it in ancient texts, even though the circumstances in which it is supposed to have been said are historical.